Lunar Edomae, a merfolk originating from the series, begins as Sun Seto's childhood friend and rival. Her troubled family history features divorced parents: an emotionally stunted father who cross-dresses and stalks her to forge connection, and a mother who abandoned her at age five. This fuels her desperate need for validation, driving her to exploit her innate mermaid singing voice as a pop idol, manipulating audiences for fame. Her polished stage identity starkly contrasts her true nature, masking profound insecurities born from maternal desertion and paternal failure.

Physically, Lunar possesses vivid pink hair and eyes, coupled with a yellow mermaid tail. She attends Isono Junior High School in its uniform. Like young merfolk, water contact involuntarily reverts her legs to a tail, requiring thorough drying to regain human form—a trait complicating her human facade.

Her personality exhibits extreme volatility, often deemed histrionic or narcissistic. She fluctuates between a crafted charming persona and a genuinely cruel, domineering disposition. This includes using plural self-references and violent outbursts, particularly toward Nagasumi Michishio, whom she tries to kill after he witnesses her mermaid form. Failing this, she declares him her servant, subjecting him to physical and emotional torment. She shows ingratitude toward the sheltering Michishio family, labeling them "slaves" despite their support. Her rivalry with Sun amplifies these traits, fueled by childhood envy over a singing contest loss and a persistent superiority-inferiority complex.

Lunar's relationships shift across the narrative. Her fixation on Nagasumi evolves from viewing him as a tool to surpass Sun toward unacknowledged romantic feelings. She moves into his home under the guise of maintaining his servitude but reveals deeper motives, including an emotional connection to Nagasumi's mother, whose resemblance to her own offers solace. Despite affection for Nagasumi, she publicly demeans him to mask vulnerability. Her dynamic with Sun remains adversarial yet hints at mutual respect; Sun tolerates Lunar's flaws, and Lunar sporadically displays genuine courtesy toward her. Acceptance by the Michishio family fosters subtle growth, making Lunar more receptive to human bonds.

Her development involves gradual humility and emotional openness. Forced into school life with Sun and Nagasumi, her public violent episodes increase, eroding her idol image. Nagasumi's unwavering kindness cultivates grudging respect that blossoms into affection, though she struggles to express it healthily. By the series' end, she remains with the Michishio family, signaling a shift from selfishness toward tentative familial belonging.

As a merfolk, Lunar shares species traits: an ultrasonic scream causing physical damage, a "War Song" inciting listener violence, and ailurophobia (fear of cats). These vocal powers anchor her idol career and combat tactics, enabling manipulation or destructive sound waves. She commands a personal army of schoolboy bodyguards, frequently battling Sun's guards, reflecting her craving for control and dominance.